Fragmentation and Mass Spectra of Alkane

• Molecular ion peak is always present although height of peak decreases as the molecular mass increases in the homologous series.
• C-C bond is weaker than C-H hence cleavage is favored at C-C bond.
• Fragmentation pattern is characterized by clusters of peak, and the corresponding peaks of each cluster are 14 mass units (CH₂) apart.
• Largest peak in each cluster represents a C_nH_2n+1 fragment and occurs at m/z= 14n+1
• Most abundant fragments are C₃ and C₄
• Fragment abundance decrease for [M- C₂H₅]⁺ and [M- CH₃]⁺is very weak or missing.
• Compounds containing more than 8 carbons show similar spectra; identify them by M⁺peak.

Branched Alkane:

• Molecular ion peak is mostly not observed.
• Bond cleavage takes place preferably at the site of branching.
• The way a molecular ion fragments depends on the strength of its bonds and the stability of the fragments.
• Generally, largest substituent is eliminated readily as a radical. The radical achieves stability by the delocalization of lone electron. 

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